To dry microorganisms, industry needs to have processes which are easy to use and which are economical. Spray-drying generally consists in spraying, in a chamber, a suspension of microorganisms in a stream of hot air, the chamber comprising, for this purpose, an inlet for heated air, an outlet for discharging air and an outlet for recovering the powder of dried microorganisms.
The spray-drying of microorganisms has, however the disadvantage of damaging, or even killing the microorganisms as the drying temperature becomes too high.
U.S. Pat. No. 3,985,901 (Instituto de Biologia Aplicada) explains, indeed, that a temperature of 180.degree. C. to 300.degree. C., at the inlet of a spraying device, is capable of killing all the live organisms. These observations are also confirmed in EP298605 (Unilever: page 2, lines 43-48), and EP63438 (Scottish Milk Marke: page 1, lines 14-21).
Some species of lactic acid bacteria are, however, naturally heat-resistant, that is to say they are capable of withstanding relatively high temperatures. Chopin et al. have thus shown that it is possible to spray-dry, at 215.degree. C., a sporulating culture of Microbacterium lacticum and to obtain slightly more than 10% survival after drying (Canadian J. Microb., 23, 755-762, 1977). Unfortunately, these species generally form part of the contaminating flora in foods which is responsible for the appearance of bad taste. These heat-resistant lactic acid bacteria are therefore not suitable for human consumption ("Fundamentals of Food Microbiology", Marion L. Fields, AVI Publishing Comp, Westport, 1979).
In conclusion, the spray-drying temperature is thus one of the factors limiting the viability of the microorganisms traditionally used in the fermentation of food products. It can in fact be noted that all the conventional processes for spray-drying microorganisms use in practice a heated air inlet temperature of the order of 100-180.degree. C., and thus reach temperatures where microorganisms are damaged or killed. Furthermore, these processes also resort to the use of protective agents to keep the dried microorganisms alive.
NL7,413,373 (DSO Pharmachim) describes a process for spray-drying cereals fermented by lactic acid bacteria in which the air inlet and outlet temperatures are 150.degree. C. and 75.degree. C. respectively.
J73008830 (Tokyo Yakult Seizo) describes a process for spray-drying microorganisms in which an air inlet temperature on the order of 120-155.degree. C., an air outlet temperature on the order of 40-55.degree. C. and protective chemical agents are used.
J57047443 (Minami Nippon Rakun) describes a similar drying process where the air inlet and outlet temperatures are on the order of 105-150.degree. C. and 55-70.degree. C. respectively.
J02086766, J02086767, J02086768, J02086769 and J02086770 (all "Kubota") describe processes for spray-drying microorganisms in which the air inlet and outlet temperatures are on the order of 110-180.degree. C. and 70-75.degree. C. respectively.
Finally, SU724113 (Kiev Bacterial Prep.), SU1097253 (Protsishin et al.), SU1227145 (Protsishin et al.), SU1292706 (Appl. Biochem. Res.) and SU1581257 (Dairyland Food Labs.) describe processes for spray-drying a bacterial culture in which the air inlet and outlet temperatures are on the order of 60-165.degree. C. and 30-75.degree. C. respectively.
It should be emphasized that limiting the drying temperature to less than 200.degree. C., during spray-drying of microorganisms, limits the yield of the process correspondingly. The objective of the present invention is to overcome this disadvantage without killing all the microorganisms in the process.